This lesson is
geared toward the 8th grade level.It assumes the student has been introduced to the basics of earthquakes,
the concepts wave propagation, P and S waves, and energy reflection
at boundaries. (A good way to do the energy reflection introduction is to
shine a flashlight at a tank of water and show the reflection on the ceiling
as well as the light at the bottom of the tank.)The students must be familiar with the basic structure of the earth
and the wave nomenclature ie. what a PP wave or a PKIKP wave is.You can tailor the degree of difficulty to your class.

The purpose of
this lesson is multifaceted.It allows the student to discover how the interior
of the earth affects the seismic waves but does so at a level they can comprehend.It allows them hands-on research using internet and actual seismograms.It also focuses them on mathematics ( D = RT ) as well as time measurement
and graphing.

Preparation:Choose a seismogram from your history
files that is particularly good at showing the various waves and reflections.If you do not have a history file go the PEPP site at Princeton University
and select any gram from their files that suits this lesson plan.Make a master copy of that portion of the gram from slightly before
the P arrival to shortly after the S arrival.Ensure the time scale is on the x-axis.Make copies for each student.For
those schools without a seismometer, go to the Princeton University PEPP site
and request this using the PEPP network key and the select a quake.Under this specific quake you may send an arrival time information
request from either your site or a selected site using the five letter code
for the site.Make copies of this for your students.

Classwork:Pass out a copy of the quake gram
and the arrival time data.Have the
students mark each wave arrival on their gram noting the exact time from their
data sheet.Have them then go to the
PEPP site at Princeton.Have them
follow your path to the arrival time request.They should put in the same site code as the one you used.At the bottom of the arrival time page is a
graphic showing how each of the waves they just marked on their gram actually
occurs.Have them reproduce the drawings
of the waves you selected for them to find on their grams.

I have found this
exercise typically takes two class periods.The first is for handing out materials, explaination of requirements
and having the students find and mark the selected waves.The second period would be for the internet research and drawing of
how those waves go through the earth.